1. Field of the Invention
The present invention relates to flat microtip display screens.
2. Discussion of the Related Art
An example of such a screen and of its addressing mode is described in U.S. Pat. No. 5,225,820 issued to Jean-Frederic Clerc.
In this screen, the cathode is comprised of a very large number of microtips connected in columns, each of which can be addressed individually. The ends of these microtips emerge in openings of an insulating grid. This grid is divided into rows which are orthogonal to the columns, and addressable individually.
An anode is placed facing the cathode/grid assembly and is separated therefrom by an empty space. On this anode are arranged groups of bands of luminescent or phosphor elements of three different colors, for example, red, green, and blue. The bands are arranged in columns parallel to the cathode columns. A group of three red, green, and blue bands has substantially the width of a cathode column. All bands of phosphors of a same color are interconnected so that it is possible to selectively address all the red bands, all the green bands or all the blue bands.
An addressing cycle of a complete image (a frame) includes the step of addressing all the anode bands of same nature, for example, all the red bands and, while these red bands are under high voltage, sequentially addressing each of the grid rows. For each biasing of a grid row, all cathode columns are addressed at potentials selected to obtain a desired luminescence of each of the red pixels. The operation is then repeated for the green bands and the blue bands and a line-by-line and color-by-color (sub-frame by sub-frame) addressing of a complete frame is thus obtained.
This addressing mode requires a switching of the potentials on the anodes. Now, the anode potential generally is a high potential so that the energy of the electrons sent by the cathodes causes a sufficient lighting of the phosphors. In the above-mentioned U.S. patent, anode potentials of around 150 volts are indicated. In practice, to obtain a sufficient lighting with conventional phosphors, potentials of around 600 to 1000 volts are conventionally used and it is desired to be able to use still higher potentials. Now, the difficulty to carry out a potential switching of an electrode increases with the potential. Thus, the need to switch high anode potentials constitutes a drawback.